The present invention relates to bit format systems for functional electrical stimulation, and more particularly, to systems for electrostimulation of the acoustic nerve.
Cochlear implants (inner ear prostheses) are a means to help profoundly deaf or severely hearing impaired persons. Unlike conventional hearing aids, which just apply an amplified and modified sound signal, a cochlear implant is based on direct electrical stimulation of the acoustic nerve. The intention of a cochlear implant is to stimulate nervous structures in the inner ear electrically in such a way that hearing impressions most similar to normal hearing are obtained.
A cochlear prosthesis essentially consists of two parts, the speech processor and the implanted stimulator. The speech processor contains the power supply (batteries) of the overall system and is used to perform signal processing of the acoustic signal to extract the stimulation parameters. The stimulator (implant) generates the stimulation patterns and conducts them to the nervous tissue by means of an electrode array which usually is positioned in the scala tympani in the inner ear. The connection between the speech processor and the implanted receiver can be established by means of encoding digital information in an rf-channel involving an inductively coupled coils system.
Decoding the information within the implant can require envelope detection. Envelope detection of an RF signal within an implant is usually performed with a simple circuit, as shown in FIG. 1, composed of a rectifier diode 4, an RC-network 1 and 2, and a comparator 7. A drawback to this circuit is that the total power consumption of the RC-network, due in part to the ohmic resistor, can be considerable when taking into account the cochlear implant application.
Stimulation strategies employing high-rate pulsatile stimuli in multichannel electrode arrays have proved to be successful in giving very high levels of speech recognition. One example therefore is the so-called xe2x80x9cContinuous Interleaved Sampling (CIS)xe2x80x9d-strategy, as described by Wilson B. S., Finley C. C., Lawson D. T., Wolford R. D., Eddington D. K., Rabinowitz W. M., xe2x80x9cBetter speech recognition with cochlear implants,xe2x80x9d Nature, vol. 352:236-238 (1991), which is incorporated herein by reference. For CIS, symmetrical biphasic current pulses are used, which are strictly non-overlapping in time. The rate per channel typically is higher than 800 pulses/sec.
Stimulation strategies based on simultaneous activation of electrode currents so far have not shown any advantage as compared to CIS. The basic problem is the spatial channel interaction caused by conductive tissue in the scala tympani between the stimulation electrodes. If two or more stimulation current sources are activated simultaneously, and if there is no correlation between them, the currents will flow between the active electrodes and do not reach the regions of neurons which are intended to be stimulated. The problem might get less severe with new stimulation electrode designs, where the electrodes are much closer to the modiolus as compared to existing electrodes, as described by Kuzma J., xe2x80x9cEvaluation of new modiolus-hugging electrode concepts in a transparent model of the cochlea,xe2x80x9d proc. 4th European Symp. on Pediatric Cochlear Implantation, xe2x80x98s-Hertogenbosch, The Netherlands (June 1998), which is incorporated herein by reference.
For high-rate pulsatile stimulation strategies, some patient specific parameters have to be determined. This is done some weeks after surgery in a so called xe2x80x9cfittingxe2x80x9d-procedure. For given phase duration of stimulation pulses and for given stimulation rate, two key parameters have to be determined for each stimulation channel:
1. the minimum amplitude of biphasic current pulses necessary to elicit a hearing sensation (Threshold Level, or THL); and
2. the amplitude resulting in a hearing sensation at a comfortable level (Most Comfort Level, or MCL).
For stimulation, only amplitudes between MCL and THL (for each channel) are used. The dynamic range between MCL and THL typically is between 6-12 dB. However, the absolute positions of MCLs and THLs vary considerably between patients, and differences can reach up to 40 dB. To cover these absolute variations, the overall dynamic range for stimulation in currently used implants typically is about 60 dB.
At the moment, MCLs and THLs are estimated during the fitting procedure by applying stimulation pulses and asking the patient about his/her subjective impression. This method usually works without problems with postlingually deaf patients. However, problems occur with prelingually or congenitally deaf patients, and in this group all agesxe2x80x94from small children to adultsxe2x80x94are concerned. These patients are usually neither able to interpret nor to describe hearing impressions, and only rough estimations of MCLs and THLs based on behavioral methods are possible. Especially the situation of congenitally deaf small children needs to be mentioned here. An adequate acoustic input is extremely important for the infant""s speech and hearing development, and this input in many cases can be provided with a properly fitted cochlear implant.
One approach for an objective measurement of MCLs and THLs is based on the measurement of the EAPs (Electrically evoked Action Potentials), as described by Gantz B., Brown C. J., Abbas P. J., xe2x80x9cIntraoperative Measures of Electrically Evoked Auditory Nerve Compound Action Potentials,xe2x80x9d American Journal of Otology 15 (2):137-144 (1994), which is incorporated herein by reference. In this approach, the overall response of the acoustic nerve to an electrical stimulus is measured very close to the position of nerve excitation. This neural response is caused by the superposition of single neural responses at the outside of the axon membranes. The amplitude of the EAP at the measurement position is between 10 xcexcV and 1000 xcexcV. Information about MCL and THL at a particular electrode position can first of all be expected from the so called xe2x80x9camplitude growth function,xe2x80x9d as described by Brown C. J., Abbas P. J., Borland J., Bertschy M. R., xe2x80x9cElectrically evoked whole nerve action potentials in Ineraid cochlear implant users: responses to different stimulating electrode configurations and comparison to psychophysical responses,xe2x80x9d Journal of Speech and Hearing Research, vol. 39:453-467 (June 1996), which is incorporated herein by reference. This function is the relation between the amplitude of the stimulation pulse and the peak-to-peak voltage of the EAP. Another interesting relation is the so called xe2x80x9crecovery functionxe2x80x9d. Here, stimulation is achieved with two pulses with varying interpulse-interval. The recovery function as the relation of the amplitude of the 2nd EAP and the interpulse-interval allows one to draw conclusions about the refractory properties and particular properties concerning the time resolution of the acoustic nerve.
In accordance with one aspect of the invention, a data transmission system having a coding unit coupled to a communications channel, that transmits encoded digital information having defined minimum and maximum durations of logical states xe2x80x9clowxe2x80x9d and xe2x80x9chighxe2x80x9d. A decoding unit is coupled to the communication channel, which receives and decodes the information. The decoder is comprised of a free running local oscillator LO coupled to an array of sampling capacitors, that effectively sample the information using the LO frequency. A circuit is coupled to the sampling capacitors, that decodes the information and corrects any mismatch between nominal and actual LO frequency. In another related embodiment, the encoded digitial information is contained in an RF signal. The data transmission system can be used in a cochlear implant system or an implantable system for functional electrostimulation.
In accordance with another embodiment of the invention, a data decoder system coupled to a communication channel that decodes information received. The decoder has a free running local oscillator LO coupled to an array of sampling capacitors, that effectively sample the information using the LO frequency. A circuit is coupled to the sampling capacitors, that decodes the information and corrects any mismatch between the nominal and actual LO frequency. The encoded digital information can be contained in an RF signal. The data decoder system can be used in a cochlear implant system, or a implantable system for functional electrostimulation.
In accordance with another emodiment of the invention, A circuit for detecting the envelope of an input signal, the circuit comprising a first sampling capacitor C1 and a second sampling capacitor C2, both capacitors coupled to ground. A first switching matrix S1 cyclically couples C1 to an input signal via a rectifier diode, the input signal being encoded with digital data; a first input of a comparator; and ground. A second switch matrix S2 cyclically couples C2 to the input signal via the rectifier diode; the first input of the comparator, and ground. A local oscillator is coupled to S1 and S2, that controls switch matrices S1 and S2, the local oscillator having period T. A dc-reference is coupled to a second input of the comparator. A flip flop is coupled to the comparator output, the flip flop being clocked by the local oscillator producing a data bit stream output indicative of the input signal""s envelope the circuit may be used for detecting the envelope of an input signal in a cochlear implant, wherein the input signal is an RF signal encoded with digital information. In a related emodiment, a first logical state is encoded in the input signal by the sequence xe2x80x9cRF-carrier offxe2x80x9d followed by xe2x80x9cRF-carrier on,xe2x80x9d and a second logical zero is encoded by the sequence xe2x80x9cRF-carrier onxe2x80x9d followed by xe2x80x9cRF-carrier off.xe2x80x9d The RF input signal may be encoded using Amplitude Shift Keying Modulation, the digital data employing a self-clocking bit format. In another related embodiment, C1 and C2 are sequentially and cyclically coupled via the switching matrices to the input signal via the rectifier diode for time duration T/2 (phase D), the comparator for time duration T (phase C), and ground for time duration T/2 (phase G). S2""s switching sequence is offset from S1""s switching sequence by a phase shift of T. The clock of the flip flop may be activated at the end of phases C on the negative slope of the local oscillator.
In accordance with another aspect of the invention, a method for data telemetry, where digital data is encoded into an input signal. The input signal is applied via a rectifier diode to a first switch matrix S1 and a second switch matrix S2, with S1 being coupled to a first sampling capacitor C1 and S2 being coupled to a second sampling capacitor C2. A local oscillator signal with period T is applied that controls S1 and S2, so as to cyclically couple C1 and C2 to the RF signal, a first input to a comparator, and ground. The comparator compares the first input to a DC reference voltage. The output of the comparator is then sampled via a flip flop clocked by the local oscillator, with the flip flop outputting a data bit stream representative of the envelope of the input signal having encoded information.
In another related embodiment, the data telemetry method detects the envelope of an input signal in a cochlear implant, the input signal is an RF encoded signal encoded with digital data. A first logical state may be encoded in the input signal by the sequence xe2x80x9cRF-carrier offxe2x80x9d followed by xe2x80x9cRF-carrier on,xe2x80x9d and a second logical state is encoded by the sequence xe2x80x9cRF-carrier onxe2x80x9d followed by xe2x80x9cRF-carrier off.xe2x80x9d The input signal can contain special bit formats, such that the signal can be switched on or off for longer durations, such as 3B/2, B being the bit duration. In another related embodiment, the RF signal is encoded using Amplitude Shift Keying Modulation, the digital data employing a self-clocking bit format. In another embodiment, the sampling capacitors C1 and C2 are sequentially and cyclically coupled via the switching matrices to the input signal for time duration T/2 (phase D), the 1st input of the comparator for time duration T (phase C), and to ground for time duration T/2 (phase G), with S2""s switching sequence being offset from S1""s switching sequence by a phase shift of T. In another embodiment, the clock of the flip flop is activated at the end of phases C on the negative slope of the local oscillator.
In another related embodiment, the data bit stream is decoded, including distinguishing four different data bit stream states, a xe2x80x9cshort lowxe2x80x9d L1 defined by a data bit stream pattern of 0 or 00, a xe2x80x9cshort highxe2x80x9d H1 defined by a data bit stream pattern of 11 or 111, a xe2x80x9clong lowxe2x80x9d L2 defined by a data bit stream pattern of 000 or 000, and a xe2x80x9clong highxe2x80x9d H2 defined by a data bit stream pattern of 1111 or 11111. Two additional bit states may be distinguished, an xe2x80x9cextra long lowxe2x80x9d L3 defined by a data bit stream pattern of 00000 or 000000, and an xe2x80x9cextra long highxe2x80x9d H3 defined by a data bit stream pattern of 111111 or 1111111. Triplet sequences may also be distinguishable, a triplet sequence having a starting short state L1 or H1, followed by a sequence of strictly alternating states L3 or H3, and a terminating short state L1 or H1. The triplet sequence can be used for control and synchronization. In another related embodiment, data telemetry is achieved by data word formats having a starting triplet sequence, followed by a particular number of information bits with self-clocking format; and a terminating triplet sequence. These data word formats can allow allow high rate stimulation strategies based on sign-correlated, simultaneous stimulation pulses. In another related embodiment, the encoded information allows stimulation with sign-correlated biphasic, symmetrical pulses, stimulation with sign-correlated triphasic, symmetrical pulses, and stimulation with sign-correlated triphasic pulses. In another embodiment of the invention, a method of employing high-rate pulsatile stimulation receives encoded information, decodes the information, and applies stimulation modes based on the decoded information. The stimulaton modes comprising sign-correlated biphasic, symmetrical pulses, sign-correlated triphasic, symmetrical pulses, and sign-correlated triphasic pulses.
In accordance with another aspect of the invention, a circuit and method for generating sign-correlated simultaneous pulsatile stimuli in a cochlear implant simultaneously applying current of same sign to a plurality of electrodes Ei. A remote ground is switched to either Vdd or ground, creating a current in the remote ground electrode equal to the sum of all single electrode Ei currents. In a related embodiment, each electrode is coupled via a switch to either a first or second current source, the second current source having the opposite sign as the first current source. In a related embodiment, the acoustic nerve is stimulated by the sign-correlated simultaneous pulsatile stimuli. The sign-correlated simultaneous pulsatile stimuli may be generated in a cochlear implant. Pulses generated can include sign-correlated biphasic, symmetrical pulses, sign-correlated triphasic, symmetrical pulses, and sign-correlated triphasic pulses.
In another embodiment, a circuit and method for measuring electrically evoked action potentials samples an input signal across a measurement electrode and a reference electrode, the measurement electrode and reference electrode being coupled in parallel. The sampled signal is then amplified and converted into a high frequency one bit sigma-delta sequence, the sequence being stored in the implant""s memory. In a related embodiment, the input signal is sampled with a first double switch. In a further related embodiment, the amplifier is a differential amplifier. The measurement electrode and the reference electrode may be coupled to the differential amplifier via coupling capacitors. In another related embodiment, the amplified analog signal is sampled and held before being digitized. In another related embodiment, the sigma-delta data sequence is transferred from memory to outside by load modulation, allowing reconstruction of the electrically evoked action potential signal from the digitized data to be achieved off-line. The method can be used in a cochlear implant.
In another embodiment of the invention, a circuit and method for measuring stimulus artifacts samples an input an input voltage across a measurement electrode and a reference electrode with a sampling capacitor to create a sampled input. At a programmable time instant, the sampled input is output to a sigma-delta modulator via a switch, to produce a sigma-delta data sequence. The sigma-delta data sequence is then sent to memory. In a related embodiment, the sigma-delta data sequence is sent from memory to outside by load modulation, allowing reconstruction of the electrically evoked action potential signal from the digitized data to be achieved off-line.